This is a descriptor. When you have some tension on the new tramp, put a ratchet around the side bars to bend them inwards (so you can see a bow) then tension the center once again (if you are on your own, use a piece of dowel in the grommets as you go to jam against the line to hold it tight when you go to the next) If you have help then they can grab the line one grommet behind you as you tighten

The lacing order is not too critical. However, I find that it's better to tension the aft lacings before the center lacing. If you do the center first, then there is a tendency to get wrinkles along the side bars/tracks because the tramp won't be able to slide in the tracks after center lacing is tightened. You also want to pull the tension on incrementally. If you just start pulling hard on the lacing line right away, you can overload a grommet and tear it out.

Typically when installing a tramp, I'll slide all three picecs into their respective tracks, then loosely lace the aft lacings, then the center (basically just thread the line but don't tighten it). Then snug up the lacings by hand- aft lacings followed by the center lacings. Finally, use a tool to pull full tension on the lacings (the tool basically consists of a T-handle with a cam cleat mounted to it which allows the use of two hands when pulling the tramp line).